Monopulse radar apparatus

ABSTRACT

A monopulse radar apparatus for automatically tracking a moving target by deriving error signals for operating the tracking circuits from composite signals formed from difference signals combined with the sum signal obtained from echo signals received within predetermined receiving patterns by means of a tracking antenna. The sum signal has a phase difference of 90* relative to the difference signals introduced into it. Each of the composite signals is fed to separate phase detecting sets, each comprising two phase detectors; the sum signal serving as a reference signal for the first phase detectors in each set and after compensation of the 90* phase difference, as a reference signal for the second phase detector in each set. The error signals are produced from the quotient of the output signals of the two phase detectors for each set.

United States Patent 1 van Popta Jan. 2, 1973 [54] MONOPULSE RADARAPPARATUS [75] Inventor: Yftinus Frederik van Popta, Hen- PrimaryExaminer Hubler gelo (0) Netherlands Attorney-Frank R. Trlfari [73]Assignee: N. V. Hollandse Signaalapparaten, 57 ABSTRACT Hengelo,Netherlands A monopulse radar apparatus for automatically [22] Fled:1969 tracking a moving target by deriving error signals for [21 App],33,250 operating the tracking circuits from composite signals formedfrom difference signals combined with the sum signal obtained from echosignals received within [30] Forelgn Apphcation Pnomy Data predeterminedreceiving patterns by means of a Jan. 3, 1969 Netherlands ..6900059tracking antenna. The sum signal has a phase difference of 90 relativeto the difference signals in- {52} US. Cl. ..343/7.4, 343/16 M troducedinto it. Each of the composite signals is fed [51] Int. Cl ..G01s 9/02to separate phase detecting sets, each comprising two [58] Field ofSearch ..343/7.4, 16 M phase detectors; the sum signal serving as areference signal for the first phase detectors in each set and afterRefelfllces Cited compensation of the 90 phase difference, as areference signal for the second phase detector in each UNITED STATESPATENTS set. The error signals are produced from the quotient 3,196,4337/1965 Barton et al ..343/7.4 of the output signals of the two phasedetectors for 2,914,762 11/1959 Gross et al. each Set. 3,243,805 3/1966Smith, Jr. ..343/7.4 3,453,617 7/1969 Begeman et a1 ..343/7.4 3 Claims,2 Drawing Figures COMPARATOR Q U 7 5 1: l 27 s TRACKING CIRCUITS c 6 l Lr, 28 I ,9 SYNC.PULSE GENERATOR ,10 TRANSMITTER LF. AMPLlFlER 11 A19 I aPHASE r-- I 1 --SENS|TIVE 1 30 l 20 DETECTORS l I 1 e --w '9OPHASE AE 3,ps H 522 l SHIFTER l i 1 F CONTROL 1 i a T 1 CIRCUITS s t Z MIXERS 1 17H I l T 31 \33 335-, I 29 SWITCHES 90 PHASE: I 32 34 es- SHIFTERT 3 1S 81 t B 2 1 T g COMPUTER l I F I i x LOCAL I 90PHASE AMPLIFIERSOSCILLATOR, 12 1 SHIFTER 3 PHASE-SENSITIVE L E J oETEcToRs R CE'VERVARIABLE 22 PHASE SHIFTERS MONOPULSE RADAR APPARATUS The inventionrelates to a monopulse radar apparatus for automatically tracking amoving target, provided with a transmitter for transmitting pulses ofhighfrequency electromagnetic energy, a receiver adapted to receive echosignals in four receiving patterns, arranged symmetrically with respectto the radar symmetry axis, and also to convert said echo signals into asum signal and two difference signals and to transform the lattersignals to intermediate frequency, said receiver being also providedwith a first, a second and a third intermediate frequency amplifier, atracking circuit being further provided per angular coordinate,controlled by an error signal whose sign and magnitude are dependent onthe amplitude and/or phase relation between the sum signal and one ofthe difference signals, said relation varying in accordance with thetarget deviation with respect to said radar symmetry axis.

In such a radar apparatus said amplitude and/or phase relation isusually realized by.means of an automatic gain control. However, thismethod is subject to deviations from the correct amplitude and/or phaserelation, the deviations being larger as the target echoes received areweaker. Said deviations are mainly due to relative differences in theintermediate frequency amplifiers.

A known solution for this problem is offered by designing the receiversuch that one and the same intermediate frequency amplifier is used forthe sum signal and the two difference signals. A receiver is known, forinstance, effecting the intermediate frequency detection of thehigh-frequency signals received by means of oscillators of variousfrequencies and feeding the signals thus obtained to one and the samebroad-band intermediate frequency amplifier. In another embodiment ofthe receiver the signals received are fed successively to one and thesame intermediate frequency amplifier, via a delay line network andafter intermediate frequency detection.

In another embodiment of the receiver the difference signals, afterintermediate frequency detection, are modulated with oscillator signalsshifted in phase through 90 with respect to each other and bothmodulated signals are added. The signal thus obtained, shifted in phaseagain through 90 is added to the intermediate frequency detected sumsignal received. Here too, one and the same intermediate frequencyamplifier will suffice.

Said methods, all aiming at a correct amplitude and/or phase relation,are based on the assumption that this desired effect can only beattained by making use of one single intermediate frequency amplifier.

However, receivers making use of one single intermediate frequencyamplifier have the drawback that a number of additional measures must betaken, which make such receivers particularly complex.

It is the object of the invention to provide a monopulse radar apparatusin which the correct amplitude and/or phase relation is obtained in asimple and easily reproducible way.

According to the invention two composite signals are obtained by addingeach of the difference signals and the sum signal with a relative phasedifference of 90;

the sum signal is fed to the first intermediate frequency amplifier,whereas said composite signals are fed to the second or thirdintermediate frequency amplifier, respectively;

each of the signals corresponding to the output signals of the secondand third intermediate frequency amplifier is fed to two phase-sensitivedetectors, a signal corresponding to the sum signal serving as areference signal for the first phase-sensitive detector, and aftercompensation of the above phase difference as a reference signal for thesecond phase-sensitive detector, and whereby the above error signal isobtained from the quotient of the output signals of the twophasesensitive detectors.

The invention and its advantages will be further explained by means ofthe figures, of which:

FIG. 1 shows a block diagram of a monopulse. radar apparatus accordingto the invention, while FIG. 2 shows a block diagram of part of a secondembodiment of a monopulse radar apparatus according to the invention.

Like parts in both figures are denoted by like numerals.

FIG. 1 shows the block diagram of a monopulse radar apparatus of thetype based on the so-called sum and difference method and used to tracka moving target. 1 denotes a transmitter, 2 a sync. pulse generator and3 a receiver. The transmitter and the receiver are connected to anantenna system 4. The energy induced in the transmitter is transmittedvia this antenna system in the pulse rhythm of the sync. pulses suppliedby the generator 2. The monopulse radar apparatus, as shown in FIG. 1,is of the type based on amplitude comparison. Accordingly, the antennasystem comprises a horn 5 divided by partitions 6 and 7 into fourquadrants opening into a comparator 8. The comparator 8, which can beconsidered to belong to the receiver, serves on the one hand for addingand subtracting, respectively, the amplitudes of the energy receivedabove and below partition 6, respectively, and on the other hand theamplitudes of the energy, received at the left and at the right ofpartition 7. Thus,-if a, b, c and d represent the respective ampiitudesof the electromagnetic energy that occur in the four parts of the hornas a result of a target echo received, this comparator supplies anelevation difference signal AE and an azimuth difference signal AB,whose respective amplitudes can be represented by:

Moreover, comparator 8 supplies a signal which is proportional to thesum of the electromagnetic energy received in each of the four quadrantsof the horn and whose amplitude can be represented by:

If, as may be assumed, the target is a point target, the signalsreceived in said horn quadrants have a relative equal phase and thedifference signals may, independent from each other, be either in phasewith or in phase opposition to the sum signal. The sign of the targetdeviation can be derived from this phase information. If, for instance,the difference signal AE is in phase with the sum signal 2, i.e.:(a-l-b) (oi-d), it follows that the target is above the symmetry-axis,whereas, if the difference signal AE is in phase opposition to the sumsignal, i.e.: (a-l-b) (cl-d), the target is below the symmetry axis. Thedifference AB can be considered in a similar way giving the sign of thedeviation in azimuth.

The signals AB, 2 and AE are fed, via waveguides 9, and 11, to receiver3, in which they are mixed with a signal coming from local oscillator12. For this purpose the receiver is provided with mixers 13, 14 and 15.Subsequently theintermediate frequency signals obtained are distributedamong three intermediate frequency amplifiers 16, 17 and 18 provided inthe receiver. The intermediate frequency signals thus amplified can beused to generate angular error signals required for controlling trackingcircuits 27 and 28 by means of which the target is tracked in azimuthand elevation.

According to the invention two composite signals are generated for thispurpose by joining the difference signal AE or AB, respectively, and thesum signal 2 with a relative phase difference of 90, the sum signal 2 isfed to intermediate frequency amplifier 17, while said composite signalsare fed to intermediate frequency amplifiers 16 and 18, respectively,and each of the signals corresponding to the output signals ofamplifiers l6 and 18 is fed to two phase-sensitive detectors 19, and 21,22 a signal corresponding to the sum signal serving as a referencesignal for the phase-sensitive detector 19 or 22, respectively, whilethis signal, after compensation of the above-mentioned 90phase-difference, serves as a reference signal for the phase-sensitivedetector 20 or 21, respectively, and whereby each of said angular errorsignals is obtained from the quotient of the output signals of bothphase-sensitive detectors 19 and 20, or 21 and 22, respectively.

For this purpose the receiver 3 is provided with a 90 phase-shiftingelement 23 in such a way that an intermediate frequency sum signal 2 isobtained that is 90 shifted in phase with respect to the obtainedintermediate frequency difference signals AE and AB Of theseintermediate frequency signals the sum signal E is fed to amplifier 17and the composite signals obtained through the joining of the sum signal2 and each of the difference signals AE and AB are fed to the amplifiers16 and 18, respectively.

If the gain factors of amplifiers 16, 17 and 18 are denoted by 01,, aand 11 respectively, the output signals of amplifiers 16, 17 and 18 canbe expressed, respectively, by:

a, 2 a2(a+b+C+d) COS (Burt The output signal of amplifier 16 is fed toboth phase sensitive detector 19 and to phase-sensitive detector 20. Theoutput signal of amplifier 17 serves as a reference signal for detector19, and after compensation of the introduced 90 phase difference, as areference signal for detector 20. Said compensation takes place by meansof a 90 phase-shifting element 24. Thus, the reference signal fordetector 20 can be represented by:

The output signal supplied by detector 19 is that component from thesignal fed by amplifier 16 that is in phase with the reference signalfed by amplifier 17. An examination of the above equations shows thatthis component is equal to:

Thus, this signal is proportional to the sum signal. The detector 20handles the signals fed to it in a way analogous to detector 19;apparently a signal is obtained that is proportional to the amplitude ofdifference signal AE, which is equal to:

It will be clear from the above that the gain factor a of amplifier 17is not at all important and that the amplitude ratio between sum signal2 and difference signal AE is also independent of the gain factor a, ofamplifier 16. Said amplitude ratio is realized by the computer 25 onreception of the required output signals from detectors 19 and 20. Theamplitude ratio obtained can be expressed by:

and gives magnitude and sign of the elevation error signal used tocontrol tracking circuit 27.

Similarly, the output signal of amplifier 18 is fed to phase-sensitivedetectors 21 and 22; the output signal of amplifier 17 serves as areference signal for the detector 22 and the outputsignal of amplifier17 asa reference signal for detector 21, after the introduced phasedifference has been compensated by means of the -90 phase-shiftingelement 26. The output signals of phase-sensitive detectors 22 and 21can be represented, respectively, by:

The amplitude ratio, determined from this by the computer, and which canbe represented by:

gives magnitude and sign of the azimuth error signal, which is used tocontrol tracking circuit 28.

In the above consideration it has been assumed that amplifiers 16, 17and 18 only differ from each other in their gain factors. However, saidamplifiers also appear to be capableof showing a different phase shift.Consequently, deviations occur from the introduced 90 phase differencebetween the sum signal and each of the difference signals. Apart fromthe above mentioned (correct output) signals the output signals ofphasesensitive detectors 19, 20 and 21,22 each contain a componentshifted through 90, which causes the amplitude ratios determined by thecomputer to differ from the above (correct) amplitude ratios. To preventsuch deviations, the monopulse radar apparatus in the embodiment shownin FIG. 1 is provided with a circuit 29, containing the variablephase-shifting elements 31 and 32, the control circuits 33 and 34 andtwo switches 35 and 36.

In the time between the reception of successive echo signals a testpulse is injected into the sum channel at the input of the receiver. Inthe embodiment shown in FIG. l a fraction of the energy transmitted isused for this purpose. This energy is fed to mixer 14 via lead 30.

The intermediate frequency signal coming from mixer 14 is distributedamong amplifiers 16, 17 and 18. If the difference in phase-shift betweenthe amplifiers 16 and 18, respectively, and amplifier 17 is indicated byand 41 respectively, the output signals .of amplifiers 16, 17 and 18 arerepresented by:

01 .4 Sin (mt+,),

a2 A (t (1 A sin (wt-H where A represents the amplitude of the energyinjected via lead 30. Now the phase-sensitive detectors 20 and 21 supplysignals whose magnitude and sign depend on the differences inphase-shift between the amplifiers. The output signals of detectors 20and 21 can be represented by:

or A sin 4n and (13 A Sin (#2 and are fed to control circuits 33 and 34,respectively, via switches 35 and 36, respectively. Said controlcircuits set the variable phase-shifting elements 31 and 32,respectively, in such a way that the output signal of amplifier 17 issubjected to a phase shift (1), with respect to the output signal ofamplifier l6 and a phase shift 4: with respect to the output signal ofamplifier 18.

In order to be able to perform such a phase control between thereception of successive echo signals, switches 35 and 36 shouldalternately supply the output signals of detectors 20 and 21 to controlcircuits 33 and 34 and computer 25. For this purpose the switches arecontrolled by sync. pulses S from generator 2.

In the embodiment of the monopulse radar apparatus as partly shown inFIG. 2, the intermediate frequency signals obtained and amplified arefed to a Doppler signal detector 37. The audio signals obtained in itcorrespond to the Doppler shift of the echo signals occurring as aresult of the target movement. The Doppler signal detector consists,according to British Pat. No. l,l56,589 and British Pat. applicationSer. No. 4,020/69, of phase-sensitive detectors 38, 39 and 40, to whichthe relevant intermediate frequency signal and a reference signal,coming from an oscillator 41 coherent with the transmitter frequency,are fed, and also of the boxcar detectors 42, 43 and 44 for stretchingthe output pulses of said detectors and Doppler filters 45, 46 and 47connected to the outputs of the boxcar detectors and from which theaudio frequency output signals are derived. The audio signals obtainedare amplified in low frequency amplifiers 48, 49 and 50.

Relative differences in the gain factors of the lowfrequency amplifiers,together with such differences occurring in the intermediate frequencyamplifiers, are eliminated from the amplitude ratios obtained in a waysimilar to that in the embodiment shown in FIG. 1. In the embodimentmeant here it is of great importance that the 90 phase-shifting element24 or 26, respectively, is so designed that the phase shift takes placepractically independent of the Doppler frequencies detected.

We would finally remark that the phase difference between the sum signaland each of the difference signals can be realized in various ways, bothby feeding the local oscillator signal to the phase-shifting element inthe way as indicated in FIG. 1, and by feeding the sum signal or each ofthe difference signals to said phase-shifting element. The phasedifference is realized in either way prior to intermediate frequencydetection. It will be clear that the phase difference can be introducedalso after intermediate frequency detection but prior to theamplification.

What we claim is:

1. Monopulse radar apparatus for automatically tracking a moving targetcomprising a high-frequency pulse transmitter, a tracking antenna toreceive echo signals within receiving patterns relative to predeterminedaxes, comparator means coupled to said antenna for converting said echosignals into a sum signal and at least one difference signal, means forprocessing said sum signal and said difference signal to produce an i f.sum signal and an i.f. difference signal, said i.f. signals having a 90phase shift relative to each other, means for combining said i.f. sumsignal and said i.f. difference signal to produce a composite i.f.signal, at least one set of first and second phase detectors forreceiving said composite i.f. signal, said first phase detector havingsaid i.f. sum signal serving as a reference signal to produce a firstoutput signal, said second phase detector having said i.f. sum signalafter compensation by a minus 90 phase shift serving as a referencesignal to produce a second output signal, computing means for producingan error signal from said first and second output signals, said errorsignals having amplitudes and polarities representing target deviationsfrom said axes, and tracking circuits controlled by said error signalsfor moving said tracking antenna in order to minimize said targetdeviations.

2. Monopulse radar apparatus as claimed in claim 1 further comprising aDoppler signal detector coupled between said combining means and saidfirst and second phase detectors for receiving said composite i.f.signal and producing by coherent means an audio sum signal and acomposite audio signal, the frequency of said signals corresponding tothe Doppler shift of the echo signals occurring as a result of targetmovement, said composite audio signal being supplied to said first andsecond phase detectors, said first phase detector having said audio sumsignal serving as a reference to produce a first output signal, saidsecond detector having said audio sum signal after compensation by aminus 90 phase shift serving as a reference to produce a second outputsignal.

3. Monopulse radar apparatus as claimed in claim 1 further comprisingmeans for injecting test pulses in the sum signal at the input to saidprocessing means in the time between the reception of successive echosignals to cause signals to be supplied by said second phase detectorsin event of deviations from the 90 phase difference between the i.f. sumsignal and the i.f. difference signal, said monopulse radar apparatusfurther comprising a variable phase-shifting element coupled betweensaid sum i.f. amplifier and said first and second phase detector andresponsive to the signals supplied by said second phase detector tocorrect said phase deviation.

mg IUNHTED emmzs @ETEHQATE RRFEQWN Patent No. 3,708,794 Dated Januarv 21973 lnvenc ro) YETINUS FREDEREIK VAN POPTA It is certified'that errorappears in the above-identified patent and that said Lettefs Patent arehereby corrected as shown below:

Column 3,, lines 53 and 58 change (a+b+c7;j;y to

Signed and sealed this 29th day of Mey 1975.-

(SEAL) Attesc;

EDWARD M.FLETCHER,JR. i I ROBERT GOTTSCHALK' Atfte'sting Officer v oCommissioner of Patents POW) STATES m' am 6) @ERTEMQATE W @QRIREEQ'EWNPatent No. 3, 705,794 Dated January 2 i973 lnventol'Q) YFTINUS FREDER IKVAN POFTA It is certifiedtnat error appoar's in the above-identifiedpatent and that saici Letters Patent are hereby corrected as shownbelow:

Column 3, lines 53 and 58 change (a-E-b-l-cfiiV to --(a+b+ i)--0 Signedand sealed this 29th day of May 1973.

(SEAL) Attest;

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK' Attestin-g Officer Commissionerof Patents

1. Monopulse radar apparatus for automatically tracking a moving targetcomprising a high-frequency pulse transmitter, a tracking antenna toreceive echo signals within receiving patterns relative to predeterminedaxes, comparator means coupled to said antenna for converting said echosignals into a sum signal and at least one difference signal, means forprocessing said sum signal and said difference signal to produce an i.f.sum signal and an i.f. difference signal, said i.f. signals having a 90*phase shift relative to each other, means for combining said i.f. sumsignal and said i.f. difference signal to produce a composite i.f.signal, at least one set of first and second phase detectors forreceiving said composite i.f. signal, said first phase detector havingsaid i.f. sum signal serving as a reference signal to produce a firstoutput signal, said second phase detector having said i.f. sum signalafter compensation by a minus 90* phase shift serving as a referencesignal to produce a second output signal, computing means for producingan error signal from said first and second output signals, said errorsignals having amplitudes and polarities representing target deviationsfrom said axes, and tracking cIrcuits controlled by said error signalsfor moving said tracking antenna in order to minimize said targetdeviations.
 2. Monopulse radar apparatus as claimed in claim 1 furthercomprising a Doppler signal detector coupled between said combiningmeans and said first and second phase detectors for receiving saidcomposite i.f. signal and producing by coherent means an audio sumsignal and a composite audio signal, the frequency of said signalscorresponding to the Doppler shift of the echo signals occurring as aresult of target movement, said composite audio signal being supplied tosaid first and second phase detectors, said first phase detector havingsaid audio sum signal serving as a reference to produce a first outputsignal, said second detector having said audio sum signal aftercompensation by a minus 90* phase shift serving as a reference toproduce a second output signal.
 3. Monopulse radar apparatus as claimedin claim 1 further comprising means for injecting test pulses in the sumsignal at the input to said processing means in the time between thereception of successive echo signals to cause signals to be supplied bysaid second phase detectors in event of deviations from the 90* phasedifference between the i.f. sum signal and the i.f. difference signal,said monopulse radar apparatus further comprising a variablephase-shifting element coupled between said sum i.f. amplifier and saidfirst and second phase detector and responsive to the signals suppliedby said second phase detector to correct said phase deviation.